1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a device and method for fabricating the LCD device having a thin film transistor (TFT).
2. Discussion of the Prior Art
Generally, an LCD device includes top and bottom glass substrates and a liquid crystal injected therebetween. On the bottom glass substrate, a plurality of gate lines extending in one direction and a plurality of data lines extending in perpendicular direction are formed. In this matrix arrangement, a plurality of TFTs are disposed near the crossover points of the data and gate lines.
On the top glass substrate, red (R), green (G) and blue (B) color filter layers and a common electrode are disposed. Generally, a light shielding layer (black matrix) is formed on the top glass substrate and a pair of polarizers are disposed on the outer surfaces of the top and bottom glass substrates to selectively transmit light.
A conventional LCD device will be described in detail below with reference to FIG. 1, which is a plan view of a conventional LCD device.
As illustrated in FIG. 1, the conventional LCD device includes a plurality of gate lines 22 formed on a transparent substrate, a plurality of data lines 24 perpendicularly crossing the gate lines 22, a plurality of TFTs “S” formed near the crossover points of the gate and data lines 22 and 24, and a plurality of pixel electrodes 14 connected to the TFTs “S”. The gate lines 22 are separated by intervals from each other and extend in one direction, whereas the data lines 24 are separated by intervals from each other and extend in a perpendicular direction to the gate lines 22. Each end portion of gate and data lines 22 and 24 has gate and data pads 21 and 23, respectively. A storage capacitor “Cst” is arranged on a predetermined portion of the gate line 22. Two adjacent gate lines 22 and two adjacent data lines 24 define the boundaries of a pixel region. In each pixel region, a TFT “S” and a pixel electrode 14 are disposed.
Each TFT includes a gate electrode 26, a source electrode 28 and a drain electrode 30. A gate insulating layer is formed between the gate and source electrodes 28 and 30 and between the gate and drain electrodes 26 and 30. The gate electrode 26 extends from the gate line 22 and the source electrode 28 extends from the data line 24. The drain electrode 30 connects the pixel electrode 14 through a contact hole 31.
The TFT transmits a signal of the data line 24 to the pixel electrode 14 in response to a signal of the gate line 22.
In the conventional LCD device having the above-described TFTs, if a signal voltage is applied to the gate electrode 26, the TFT is turned on so as to transmit a data voltage representing picture data to the pixel electrode 14 and the liquid crystal.
FIGS. 2A to 2E show fabrication process steps of an active matrix liquid crystal display device according to the conventional art.
First, a first metal layer is deposited on a substrate 1 by a sputtering process after a cleaning process in order to remove organic materials and alien substances from the substrate 1, thereby enhancing adhesion between the substrate 1 and the metal layer. FIG. 2A shows a step for forming a gate electrode 26 and a first capacitor electrode 22 by patterning the first metal layer using a first mask.
A low resistance metal such as aluminum is used to form the gate electrode 26 so as to reduce the RC delay. However, pure aluminum has weak resistance to most enchants and may result in line defects due to a formation of a hillock during a high temperature process. Thus, an aluminum alloy is used. And in some cases, a double layered gate is used wherein another metal layer covers the aluminum or aluminum alloy.
A gate insulating layer 50 is deposited on the whole surface of the substrate 1 covering the gate and capacitor electrodes 26 and 22. Then, a pure amorphous silicon (a-Si:H) layer 52 and a doped amorphous silicon (n+ a-Si:H) layer 54 are deposited sequentially on the gate insulating layer 50.
As shown in FIG. 2B, an active layer 55 and a semiconductor island 53 are formed by patterning the silicon layers 52 and 54 using a second mask. The doped amorphous silicon layer 54 (i.e. ohmic contact layer) reduces the contact resistance between the active layer 55 and an electrode which is formed later.
FIG. 2C shows a step for forming a data line 24, source and drain electrodes 28 and 30 by depositing a second metal layer. At the same time, a second capacitor electrode 58 is formed on the gate insulating layer 50, covering a portion of the first capacitor electrode 22.
Then, the ohmic contact layer between the source and drain electrodes 28 and 30 is etched using the source and drain electrodes 28 and 30 as a mask.
As depicted in FIG. 2D, an insulating layer is deposited on the entire surface of the substrate 1 covering the source and drain electrodes 28 and 30. The insulating layer is patterned using a fourth mask to form a protection film 56. The protection film 56 may be selected from inorganic materials such as SiNx and SiO2 or organic materials such as a BCB (benzocyclobutene). In addition, a material having a high light transmittance, humidity resistance and durability is used to form the protection film 56 in order to protect the channel area of the TFT and major portions of a pixel region from possible exposure to humidity and scratches during later processing steps.
Further, a data pad contact hole 33 is formed on the data pad 23, and drain and capacitor contact holes 31 and 59 are formed on the drain electrode 30 and the second capacitor electrode 58, respectively.
FIG. 2E shows a step for forming a pixel electrode 14 by depositing a transparent conducting oxide (TCO) layer 15 and patterning it using a fifth mask. Indium tin oxide (ITO) is usually employed for the transparent conducting oxide layer. The pixel electrode 14 contacts the second capacitor electrode 58 through the capacitor contact hole 59 and the drain electrode 30 through the drain contact hole 31. Another portion of the transparent conducting oxide layer 15 is also formed contacting the data pad 23 through the data pad contact hole 33.
As described, the conventional art requires at least five masks in fabricating the TFT array panel of the LCD device, and each mask process requires many steps such as cleaning, depositing, baking and etching. Therefore, if the number of mask processes is reduced, even if only by one, then production would be increased and cost would be decreased.